In this article, near-infrared dyes of different structures have been investigated as new photoinitiators/photosensitizers for the free radical polymerization (FRP) of methacrylates upon Near InfraRed (NIR) light exposure using a laser diode @785 nm, @940 nm and @1064 nm. Interestingly, the use of squaraine, squarylium, boron-pyrromethene and porphyrin derivatives as efficient photoinitiators is clearly highlighted. These dyes are used in combination with an iodonium salt and a phosphine. Additionally, a thermal initiator can be added to the resin to take advantage of the photothermal effect, i.e., to the heat release during the polymerization process, so that polymerization kinetics could be greatly improved compared to the pure photochemical mode. These dyes can be proposed as alternatives to the well-established cyanine dyes, more commonly used for the photopolymerization of (meth)acrylates under NIR light.

The design and development of high-performance photoinitiating systems applicable to visible light delivered from light-emitting diodes (LEDs) attract ever-increasing attention due to their great potential applications in various fields. Chalcones belong to a well-established class of natural compounds present in many plants. In this context, photoinitiators based on the scaffolds of natural compounds have probably great potential to open the way for non-toxic photoinitiators in the manufacturing of biocompatible polymer materials. In this work, 23 different chalcones selected from in silico molecular design (i.e. through molecular orbital calculations) were used in conjunction with an amine i.e. ethyl 4-(dimethylamino) benzoate (EDB) and an iodonium salt (bis-(4-tert-butylphenyl)iodonium hexafluorophosphate, Iod) to form photoinitiating systems (PISs) capable of initiating the polymerization of polyethylene glycol (600) diacrylate (PEG-diacrylate) via an oxidation–reduction reaction mechanism. The formation of hydrogels by the photopolymerization of PEG-acrylate is found to be very efficient. Remarkably, 10 of the 23 chalcones were completely new and even never synthesized prior to this work (chalcones 4, 5, 7, 8, 9, 10, 12, 15, 16 and 21). By using computational quantum chemistry, fluorescence, steady-state photolysis, and electron spin resonance spin-trapping techniques, the photoinitiation mechanism of three-component photoinitiating systems based on chalcones/Iod/EDB is clarified. The results demonstrate that several chalcone-based PISs can initiate free radical photopolymerization. Moreover, the developed chalcone-based three-component systems are applied to generate 3D hydrogel patterns using the 3D laser write technology. This research provides some novel insights into photopolymerization reactions with three-component photoinitiating systems used for 3D printing of hydrogels.